Device for on-line control of output power of vacuum-UV laser

ABSTRACT

A beam delivery system for a laser emitting at a relevant wavelength of less than 200 nm is provided. The system includes a sealed enclosure connected to the laser and surrounding the path of the beam as it exits the laser resonator. The enclosure extends between the laser output coupler and a photodetector sensitive at the wavelength of the relevant laser emission. The interior of the enclosure, and thus the beam path between the output coupler and the detector, is substantially free of species that strongly photoabsorb radiation at the relevant laser emission wavelength. A beam splitting element diverts at least a portion of the beam for measurement by the detector. The beam splitting element preferably includes a beam splitting mirror, holographic beam sampler or diffraction grating. In addition, optics are preferably provided for filtering a visible portion of the diverted beam, so that substantially only a VUV portion of the diverted beam is received at the detector. The filtering optics preferably include a diffraction grating, holographic beam sampler or one or more dichroic mirrors.

PRIORITY

[0001] This application is a divisional application filed under 37C.F.R. 1.53(b) which claims the benefit of priority to U.S. patentapplication Ser. No. 09/598,522, filed Jun. 21, 2000, which claims thebenefit of priority to U.S. provisional patent application Ser. No.60/140,530, filed Jun. 23, 1999, which is hereby incorporated byreference, and which is also a Continuation-in-Part of U.S. patentapplication Ser. No. 09/343,333, filed Jun. 30, 1999, now U.S. Pat. No.6,219,368, which claims the benefit of priority to U.S. provisionalpatent application Ser. No. 60/119,973, filed Feb. 12, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to on-line control of the output power of amolecular fluorine laser beam, and particularly to a technique forredirecting VUV light of the beam to a VUV detector, while filteringvisible light from the redirected beam.

[0004] 2. Discussion of the Related Art

[0005] The molecular fluorine laser emitting at 157 nm has anadvantageously short wavelength, or high photon energy. Because of this,very small structures, such as sub-0.18 micron structures and evensub-0.10 micron structures, may be formed by photolithographic exposureon semiconductor substrates. TFT annealing and micromachiningapplications may also be performed advantageously at this wavelength.

[0006] For the applications mentioned above, on-line monitoring andcontrol of the output power of the laser may be advantageously performedsuch that the energy stability of the output beam and overallperformance of the laser may be enhanced. For this purpose, an energy orpower detector may be configured to receive a split off portion of theoutput beam. The input voltage and other conditions such as the gasmixture composition may be actively adjusted depending on the measuredpulse energy, energy dose or moving average energy in order to providehigh stability.

[0007] There are several factors inhibiting use of conventional lightdetectors for on-line monitoring of VUV laser output. First, laserradiation below 200 nm is strongly absorbed in the atmosphere, e.g., bywater vapor, oxygen, hydrocarbons, and fluorocarbons. Specifically, at157 nm, the extinction length of a molecular fluorine laser beam isaround 1 mm or less in ambient air due mostly to the presence of oxygenand water vapor in the air. Second, contaminants such as oil vapors andother organic substances generated, for instance, by vacuum pumps andplastic enclosures tend to form films on optical surfaces causing strongabsorption. Third, the molecular fluorine laser generates, in additionto 157 nm light, radiation in the red part of the visible spectrum,between 600 and 800 nm, due to emission by excited atomic fluorinespecies in the laser gas mixture. This red emission is sensed by mostoptical detectors whose sensitivity tends to be higher in the visiblepart of the spectrum, as compared to that in the VUV range, i.e., at 157nm.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the invention to provide a methodand apparatus for detecting output power of a molecular fluorine laserbeam without the beam being substantially absorbed as it propagates tothe detector.

[0009] It is a further object of the invention to provide a method andapparatus for detecting the VUV output of a molecular fluorine laserwhile any accompanying visible output of the laser is substantiallysuppressed before reaching the detector.

[0010] In accord with the above objects, a beam delivery system for alaser emitting at a relevant wavelength of less than 200 nm is provided.The system includes a sealed enclosure surrounding the path of the beamas it exits the laser resonator. The enclosure extends between the laseroutput coupler and a photodetector sensitive at the wavelength of therelevant laser emission. The interior of the enclosure, and thus thebeam path between the output coupler and the detector, is substantiallyfree of species that strongly photoabsorb radiation at the relevantlaser emission wavelength. A beam splitting element diverts at least aportion of the beam for measurement by the detector.

[0011] The beam splitting element preferably includes a beam splittingmirror, holographic beam sampler or diffraction grating. In addition,optics are preferably provided for filtering a visible portion of thediverted beam, so that substantially only a VUV portion of the divertedbeam is received at the detector. The filtering optics preferablyinclude a diffraction grating, holographic beam sampler or dichroicmirrors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 schematically illustrates a molecular fluorine laser systemin accord with a preferred embodiment.

[0013]FIG. 2 schematically illustrates a beam path enclosure in accordwith a first preferred embodiment.

[0014]FIG. 3 shows plots of measured laser output power versus time fora molecular fluorine laser system including a beam path enclosure havingan evacuated interior and an enclosure purged with a steady flow ofinert gas in accord with a preferred embodiment.

[0015]FIG. 4 schematically illustrates a beam path enclosure in accordwith a second preferred embodiment.

[0016]FIG. 5 schematically illustrates a beam path enclosure in accordwith a third preferred embodiment.

[0017]FIG. 6 schematically illustrates a beam path enclosure in accordwith a fourth preferred embodiment.

[0018]FIGS. 7a and 7 b schematically illustrate alternative beamsplitter configurations to the first and third preferred embodiments ofFIGS. 2 and 5, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The preferred embodiments described below provide means ofon-line monitoring of the output power of a vacuum UV laser,specifically a molecular fluorine laser, operating in a wavelength rangebelow 200 nm. Preferred and alternative embodiments described belowfurther provide means of minimizing variations of sensitivity of VUVlaser energy monitor due to absorption, as well as suppressing a visiblered portion of the output. The former is generally achieved by providinga hermetic enclosure which is preferably purged with an inert gas. Thelatter is preferably provided by one of three techniques including theuse of a diffraction grating, a dichroic thin-film dielectric mirrorarrangement, or a holographic beam sampler.

[0020] Referring to FIG. 1, a VUV laser system, preferably a molecularfluorine laser for deep ultraviolet (DUV) or vacuum ultraviolet (VUV)lithography, is schematically shown. Alternative configurations forlaser systems for use in such other industrial applications as TFTannealing and/or micromachining, e.g., are understood by one skilled inthe art as being modified from the system shown in FIG. 1 to meet therequirements of that application. For this purpose, alternative VUVlaser system and component configurations are described at U.S. patentapplications Ser. Nos. 09/317,695, 09/317,526, 09/317,527, 09/343,333,60/122,145, 60/140,531, 60/162,735, 60/166,952, 60/171,172, 60/141,678,60/173,993, 60/166,967, 60/172,674, and 60/181,156, and U.S. patentapplication of Kleinschmidt, serial number not yet assigned, filed May18, 2000, for “Reduction of Laser Speckle in Photolithography byControlled Disruption of Spatial Coherence of Laser Beam,” and U.S. Pat.No. 6,005,880, each of which is assigned to the same assignee as thepresent application and is hereby incorporated by reference.

[0021] The system shown in FIG. 1 generally includes a laser chamber 2having a pair of main discharge electrodes 3 connected with asolid-state pulser module 4, and a gas handling module 6. Thesolid-state pulser module 4 is powered by a high voltage power supply 8.The laser chamber 2 is surrounded by optics module 10 and optics module12, forming a resonator. The optics modules 10 and 12 are controlled byan optics control module 14.

[0022] A computer 16 for laser control receives various inputs andcontrols various operating parameters of the system. A diagnostic module1 8 receives and measures various parameters of a split off portion ofthe main beam 20 via optics for deflecting a small portion of the beamtoward the module 18, such as preferably a beam splitter module 21, asshown. The beam 20 is preferably the laser output to an imaging system(not shown) and ultimately to a workpiece (also not shown). The lasercontrol computer 16 communicates through an interface 24 with astepper/scanner computer 26 and other control units 28.

[0023] The laser chamber 2 contains a laser gas mixture and includes apair of main discharge electrodes and one or more preionizationelectrodes (not shown). Preferred main electrodes 3 are described atU.S. patent applications Nos. 09/453,670, 60/184,705 and 60/128,227,each of which is assigned to the same assignee as the presentapplication and is hereby incorporated by reference. Other electrodeconfigurations are set forth at U.S. Pat. Nos. 5,729,565 and 4,860,300,each of which is assigned to the same assignee, and alternativeembodiments are set forth at U.S. Pat. Nos. 4,691,322, 5,535,233 and5,557,629, all of which are hereby incorporated by reference. The laserchamber 2 also includes a preionization arrangement (not shown).Preferred preionization units are set forth at U.S. patent applicationsSer. Nos. 60/162,845, 60/160,182, 60/127,237, 09/535,276 and 09/247,887,each of which is assigned to the same assignee as the presentapplication, and alternative embodiments are set forth at U.S. Pat. Nos.5,337,330, 5,818,865 and 5,991,324, all of the above preionization unitsbeing hereby incorporated by reference.

[0024] The solid-state pulser module 14 and high voltage power supply 8supply electrical energy in compressed electrical pulses to thepreionization and main electrodes within the laser chamber 2 to energizethe gas mixture. The preferred pulser module and high voltage powersupply are described at U.S. patent applications Ser. Nos. 60/149,392,60/198,058, and 09/390,146, and U.S. patent application of Osmanow, etal., serial number not yet assigned, filed May 15, 2000, for “ElectricalExcitation Circuit for Pulsed Laser”, and U.S. Pat. Nos. 6,005,880 and6,020,723, each of which is assigned to the same assignee as the presentapplication and which is hereby incorporated by reference into thepresent application. Other alternative pulser modules are described atU.S. Pat. Nos. 5,982,800, 5,982,795, 5,940,421, 5,914,974, 5,949,806,5,936,988, 6,028,872 and 5,729,562, each of which is hereby incorporatedby reference. A conventional pulser module may generate electricalpulses in excess of 3 Joules of electrical power (see the '988 patent,mentioned above).

[0025] The laser resonator which surrounds the laser chamber 2containing the laser gas mixture includes optics module 1 0 includingline-narrowing optics for a line narrowed excimer or molecular fluorinelaser, which may be replaced by a high reflectivity mirror or the likeif line-narrowing is not desired. Exemplary line-narrowing optics of theoptics module 10 include a beam expander, an optional etalon and adiffraction grating, which produces a relatively high degree ofdispersion, for a narrow band laser such as is used with a refractive orcatadioptric optical lithography imaging system. For a semi-narrow bandlaser such as is used with an all-reflective imaging system, the gratingis replaced with a highly reflective mirror, and a lower degree ofdispersion may be produced by a dispersive prism.

[0026] The beam expander of the line-narrowing optics of the opticsmodule 10 typically includes one or more prisms. The beam expander mayinclude other beam expanding optics such as a lens assembly or aconverging/diverging lens pair. The grating or highly reflective mirroris preferably rotatable so that the wavelengths reflected into theacceptance angle of the resonator can be selected or tuned. The gratingis typically used, particularly in KrF and ArF lasers, both forachieving narrow bandwidths and also often for retroreflecting the beamback toward the laser tube. One or more dispersive prisms may also beused, and more than one etalon may be used.

[0027] Depending on the type and extent of line-narrowing and/orselection and tuning that is desired, and the particular laser that theline-narrowing optics of the optics module 10 is to be installed into,there are many alternative optical configurations that may be used. Forthis purpose, those shown in U.S. Pat. Nos. 4,399,540, 4,905,243,5,226,050, 5,559,816, 5,659,419, 5,663,973, 5,761,236, and 5,946,337,and U.S. patent applications Ser. Nos. 09/317,695, 09/130,277,09/244,554, 09/317,527, 09/073,070, 60/124,241, 60/140,532, 60/147,219and 60/140,531, 60/147,219, 60/170,342, 60/172,749, 60/178,620,60/173,993, 60/166,277, 60/166,967, 60/167,835, 60/170,919, 60/186,096,each of which is assigned to the same assignee as the presentapplication, and U.S. Pat. Nos. 5,095,492, 5,684,822, 5,835,520,5,852,627, 5,856,991, 5,898,725, 5,901,163, 5,917,849, 5,970,082,5,404,366, 4,975,919, 5,142,543, 5,596,596, 5,802,094, 4,856,018,5,970,082, 5,978,409, 5,999,318, 5,150,370 and 4,829,536, are eachhereby incorporated by reference into the present application.

[0028] Optics module 12 preferably includes means for outcoupling thebeam 20, such as a partially reflective resonator reflector. The beam 20may be otherwise outcoupled such as by an intraresonator beam splitteror partially reflecting surface of another optical element, and theoptics module 12 would in this case include a highly reflective mirror.The optics control module 14 controls the optics modules 10 and 12 suchas by receiving and interpretting signals from the processor 16, andinitiating realignment or reconfiguration procedures (see the '241,'695, 277, 554, and 527 applications mentioned above).

[0029] The laser chamber 2 is sealed by windows transparent to thewavelengths of the emitted laser radiation 14. The windows may beBrewster windows or may be aligned at another angle to the optical pathof the resonating beam. The beam path between the laser chamber and eachof the optics modules 10 and 12 is sealed by enclosures 17 and 19, andthe interiors of the enclosures is substantially free of water vapor,oxygen, hydrocarbons, fluorocarbons and the like which otherwisestrongly absorb VUV laser radiation.

[0030] After a portion of the output beam 20 passes the outcoupler ofthe optics module 12, that output portion impinges upon beam splittermodule 21 which includes optics for deflecting a portion of the beam tothe diagnostic module 1 8, or otherwise allowing a small portion of theoutcoupled beam to reach the diagnostic module 18, while a main beamportion 20 is allowed to continue as the output beam 20 of the lasersystem. Preferred optics include a beamsplitter or otherwise partiallyreflecting surface optic. The optics may also include a mirror or beamsplitter as a second reflecting optic. More than one beam splitterand/or HR mirror(s), and/or dichroic mirror(s) may be used to directportions of the beam to components of the diagnostic module 18. Aholographic beam sampler, transmission grating, partially transmissivereflection diffraction grating, grism, prism or other refractive,dispersive and/or transmissive optic or optics may also be used toseparate a small beam portion 22 from the main beam 20 for detection atthe diagnostic module 18, while allowing most of the main beam 20 toreach an application process directly or via an imaging system orotherwise. The output beam 20 may be transmitted at the beam splittermodule while a reflected beam portion 22 is directed at the diagnosticmodule 18, or the main beam 20 may be reflected, while a small portion22 is transmitted to the diagnostic module 1 8. The portion of theoutcoupled beam which continues past the beam splitter module 21 is theoutput beam 20 of the laser, which propagates toward an industrial orexperimental application such as an imaging system and workpiece forphotolithographic applications.

[0031] An enclosure 23 seals the beam path of the beams 22 and 20 suchas to keep the beam paths free of photoabsorbing species. The enclosure23 and beam splitting module 21 will be described in more detail belowwith respect to FIGS. 2-7. For example, the beam splitting module 21preferably also includes optics for filtering visible red light from thebeam 22 so that substantially only VUV light is received at a detectorof the diagnostic module 18. Also, an inert gas purge is preferablyflowing through the enclosure 23.

[0032] The diagnostic module 18 preferably includes at least one energydetector. This detector measures the total energy of the beam portionthat corresponds directly to the energy of the output beam 20. Anoptical configuration such as an optical attenuator, e.g., a plate or acoating, or other optics may be formed on or near the detector or beamsplitter module 21 to control the intensity, spectral distributionand/or other parameters of the radiation impinging upon the detector(see U.S. patent applications Ser. Nos. 09/172,805, 60/172,749,60/166,952 and 60/178,620, each of which is assigned to the sameassignee as the present application and is hereby incorporated byreference).

[0033] One other component of the diagnostic module 18 is preferably awavelength and/or bandwidth detection component such as a monitor etalonor grating spectrometer (see U.S. patent applications Ser. Nos.09/416,344, 60/186,003, 60/158,808, and 60/186,096, and Lokai, et al.,serial number not yet assigned, “Absolute Wavelength Calibration ofLithography Laser Using Multiple Element or Tandem See Through HollowCathode Lamp”, filed May 10, 2000, each of which is assigned to the sameassignee as the present application, and U.S. Pat. Nos. 4,905,243,5,978,391, 5,450,207, 4,926,428, 5,748,346, 5,025,445, and 5,978,394,all of the above wavelength and/or bandwidth detection and monitoringcomponents being hereby incorporated by reference.

[0034] Other components of the diagnostic module may include a pulseshape detector or ASE detector, such as are described at U.S. patentapplications Ser. Nos. 09/484,818 and 09/418,052, respectively, each ofwhich is assigned to the same assignee as the present application and ishereby incorporated by reference, such as for gas control and/or outputbeam energy stabilization. There may be a beam alignment monitor, e.g.,such as is described at U.S. Pat. No. 6,014,206 which is herebyincorporated by reference.

[0035] The processor or control computer 16 receives and processesvalues of some of the pulse shape, energy, amplified spontaneousemission (ASE), energy stability, energy overshoot for burst modeoperation, wavelength, spectral purity and/or bandwidth, among otherinput or output parameters of the laser system and output beam. Theprocessor 16 also controls the line narrowing module to tune thewavelength and/or bandwidth or spectral purity, and controls the powersupply and pulser module 4 and 8 to control preferably the movingaverage pulse power or energy, such that the energy dose at points onthe workpiece is stabilized around a desired value. In addition, thecomputer 16 controls the gas handling module 6 which includes gas supplyvalves connected to various gas sources.

[0036] The laser gas mixture is initially filled into the laser chamber2 during new fills. The gas composition for a very stable excimer laserin accord with the preferred embodiment uses helium or neon or a mixtureof helium and neon as buffer gas, depending on the laser. Preferred gascomposition are described at U.S. Pat. Nos. 4,393,405 and 4,977,573 andU.S. patent applications Ser. Nos. 09/317,526, 09/513,025, 60/124,785,09/418,052, 60/159,525 and 60/160,126, each of which is assigned to thesame assignee and is hereby incorporated by reference into the presentapplication. The concentration of the fluorine in the gas mixture mayrange from 0.003% to 1.00%, and is preferably around 0.1%. An additionalgas additive, such as a rare gas, may be added for increased energystability and/or as an attenuator as described in the '025 application,mentioned above. Specifically, for the F2-laser, an addition of Xenonand/or Argon may be used. The concentration of xenon or argon in themixture may range from 0.0001% to 0.1%. For an ArF-laser, an addition ofxenon or krypton may be used also having a concentration between 0.0001%to 0.1%.

[0037] Halogen and rare gas injections, total pressure adjustments andgas replacement procedures are performed using the gas handling module 6preferably including a vacuum pump, a valve network and one or more gascompartments. The gas handling module 6 receives gas via gas linesconnected to gas containers, tanks, canisters and/or bottles. Preferredgas handling and/or replenishment procedures of the preferredembodiment, other than as specifically described herein, are describedat U.S. Pat. Nos. 4,977,573 and 5,396,514 and U.S. patent applicationsSer. Nos. 60/124,785, 09/418,052, 09/379,034, 60/171,717, and60/159,525, each of which is assigned to the same assignee as thepresent application, and U.S. Pat. Nos. 5,978,406, 6,014,398 and6,028,880, all of which are hereby incorporated by reference. A Xe gassupply may be included either internal or external to the laser systemaccording to the '025 application, mentioned above.

[0038] Referring now to FIG. 2, a first preferred embodiment of a beamdelivery system includes the enclosure 23, mentioned briefly above,which seals the beam paths of the beams 20 and 22 everywhere after thebeam is outcoupled from the laser system until the beam 20 reaches theapplication process 30. The enclosure 23 is maintained substantiallyfree of VUV photoabsorbing species such as water vapor, oxygen,hydrocarbons and fluorocarbons preferably by a method as set forth atU.S. patent application Ser. No. 09/343,333, incorporated by referenceabove.

[0039] Briefly, the preferred method, as described in more detail in the'333 application, is a method wherein the enclosure 23 is first pumpeddown to a rough vacuum, e.g., using a mechanical roughing pump, such asa rotary vane pump. Next, an inert gas is purged into the enclosure 23.The vacuum/purging steps are preferably performed some optimal number oftimes, such as from one to ten times, balancing the removal ofphotoabsorbing impurities in the enclosure 23 with the time it takes toperform those steps. Then, an inert gas is flowed at a slightoverpressure (e.g., <50 mbar) using gas inlets 32 a and 32 b and gasoutlet 34. The VUV laser is operated with the inert gas flowing at theslight overpressure. The above method is preferred as being time andcost efficient. Two alternative methods which may, however, be used forkeeping the beam path substantially free of photoabsorbing species arepumping the enclosure 23 to high vacuum, and flowing an inert gas athigh flow rate through the enclosure 23.

[0040] The application process 30 may include a separate housing for theworkpiece and/or additional optical equipment such as an imaging system,or may be the workpiece itself. Two reflectors 36 a and 36 b, preferablyboth being beam splitters or one reflector 36 a being a beam splitterand the other reflector 36 b being a mirror, are shown for splitting offbeam 22 and allowing the substantial portion of the beam 20 to passthrough unhindered towards the application process 30. The beam 22 isdirected ultimately to the detector 38, preferably via a collectinglens, grids and a diffuser (collectively 40), and a signal 42corresponding to the energy measured is sent to a processor (not shown)or other data acquisition equipment using a vacuum feedthrough 44. Avisible red light portion of the beam 22 is preferably first filteredsuch that substantially only the VUV portion of the beam 22 reaches thedetector 38, as described in more detail below.

[0041] The reflectors 36 a and 36 b preferably each comprise uncoatedplates made of excimer grade CaF₂, MgF₂ quartz, fused silica, dopedfused silica, LiF, BaF₂, or other material that is mostly transparent toVUV radiation. In this case, the reflectivity of each reflector 36 a and36 b is preferably approximately 3-15%, e.g., 8%. Additional dielectriccoatings can be deposited onto preferred reflectors 36 a and 36 b inorder to reduce or increase reflectivity. However, uncoated surfacesallow the preferred reflectors 36 a and 36 b to have longer lifetimesthan those with coated surfaces.

[0042] The incidence angles of the beam onto the preferred reflectors 36a and 36 b are preferably relatively small, in order to reduce thedependence of the reflectivity on the polarization of the incident laserbeam, as explained below. The reflectivity of the uncoated surface forp- and s-polarized beams is described by Fresnel's formulas:

R _(s)=sin²(φ−φ′)/sin²(φ+φ′),

R _(p)=tan²(φ−φ′)/tan²(φ+φ′),

[0043] where incident and refracted angles φ and φ′ are approximatelyrelated through the formula:

sin(=n•sin(φ′),

[0044] where n is the refractive index of the material.

[0045] Thus, for the angles that approach Brewster's angleφ_(B)=arctan(n), the reflectivity of the p-component decreases to zero,while s-components experience an increase in reflectivity. For example,for materials such as CaF₂ or MgF₂ with refractive indices ofapproximately 1.5, Brewster's angle φ_(B) is approximately 56°. At 45°incidence, the ratio of reflectivities for s- and p-polarized beams isstill as high as 10.5. One should preferably avoid such contrast sincein the case of p-polarized laser output, small changes of polarizationstate can cause large errors in energy readings. Therefore, theincidence angles are preferably limited to less than 22.5°.

[0046] The reflectors 36 a and 36 b direct the beam 22 at an appropriateangle to the diffraction grating 46. The grating 46 shown is areflection grating 46. An alternative configuration may include atransmission grating. A grism may also alternatively be used preferablymade of CaF₂ or another of the VUV transparent materials set forthabove.

[0047] The grating 46 provides separation of the VUV beam from the redportion of the beam 22. The incidence and reflection angles θ_(i) andθ_(r) into/from the diffraction grating 46 are related through theformula:

sin(θ₁)−sin(θ_(r))=mλ/d

[0048] where λ is the wavelength, m is diffraction order (m=0, ±1, ±2 .. . ) and d is the periodic spacing of the grooves of the grating. Forexample, a typical grating with the groove density of 1200 grooves/mmand an incidence angle of 11°, zeroth- and first-order reflected beamsat 157 nm will be at −11° and zero°, respectively. At the same time, thenearest angles of reflection for the red light of the wavelength ofapproximately 700 nm will be around −11° and 40.5° for zeroth- and-first orders, respectively. Thereafter, one can separate the VUV andred portions of the beam 22 by using an aperture in front of thedetector as shown in FIGS. 2, 4, 5 or 6.

[0049] Collecting lens and diffuser, of the assembly 40 which alsoincludes grids, described below, should be preferably made of one of thematerials mentioned above as a choice for preferred beam-splitters 36 aand 36 b. The diffuser serves to attenuate the beam and also to decreasedependence of the overall sensitivity on the beam alignment. Theattenuator grids are preferably fine-pitch stainless steel meshes. Theseserve as additional diffusers and attenuators, and additionally provideshielding of the detector against electromagnetic interference.Additional beam shaping optics, such as an aperture 47, may be includes,e.g., as set forth at U.S. patent application Ser. No. 60/172,749, whichis assigned to the same assignee and is hereby incorporated byreference.

[0050] Preferably, optical components 40 and detector 38 are encasedinto the enclosure 23, as shown, or in a separately hermetically sealedhousing with inert gas purging, having an entrance window for the beam22. It has been observed that when such enclosure is evacuated, theretends to occur a build-up of hydrocarbon film on the optical elementsexposed to the UV beam. This is likely caused by polymerization oforganic molecules present in low-grade vacuum. Instead of providinghigh-vacuum enclosure, it is preferred to arrange purging, as describedabove (see the '333 application) with clean inert gas (such as nitrogen,helium, argon, neon and others) at a flow rate preferably around 5liters/min or less.

[0051] Experimentally, it has been observed that purging improvesstability of the laser output by at least an order of magnitude, asshown in FIG. 3. FIG. 3 shows the output power of a laser in accord withthe preferred embodiment of FIGS. 1 and 2. Plot 1 shows the output powerwhen inert gas purging is used, and plot 2 shows the output power whenan evacuated housing is used. Plot 1 shows the output power stabilizedaround 2.2 W over about 2.5 hours, while plot 2 shows the output powerdecreasing from around 2.8 W to around 2.5 W over the same period. Thusthe energy stability observed with inert gas purging is far better thanwith an evacuated housing.

[0052] The gas flow path is also preferably arranged in such a way as tominimize or avoid any “dead”, un-purged spaces in the enclosure 23 ofFIG. 2. For example, an additional gas inlet 32 b is preferably providedas shown in FIG. 2 to the chamber encasing the detector and separatedfrom the main beam path by grid attenuators. The collecting lens anddiffuser are preferably mounted so that there are vent holes aroundthem. Among mentioned above inert gases, it is preferred to useultra-high purity argon, for the reason of its relatively low cost, ascompared to helium and neon. Nitrogen of ultra-high purity gradetypically contains higher levels of impurities as compared to UHP-heliumand neon and, therefore, is less suitable for purging.

[0053] The detector 38 may be one of, but is not limited to, a siliconphotodiode, pyroelectric, thermopile, electron phototube,photomultiplier, CCD-detector, or diamond detector as set forth in U.S.patent application Ser. No. 60/122,145, which is assigned to the sameassignee as is hereby incorporated by reference. Preference is based onthe lifetime, sensitivity, time resolution and cost.

[0054] The diffraction grating 46 is preferably aluminum-coated andprotected with the thin layer of MgF₂, and may be otherwise as may beknown to one skilled in the art of UV diffraction gratings. The gratingmay be one of those described at U.S. patent application Ser. No.60/167,835, which is assigned to the same assignee, and U.S. Pat. No.5,999,318, each of which is hereby incorporated by reference. Sides ofthe grating should be carefully protected from stray UV light byappropriate shields, for example made of aluminum foil. The purpose ofthe shields is to prevent degradation and outgassing of organicmaterials beneath the aluminum layer which are commonly used in theprocess of replication of gratings.

[0055] Referring to FIG. 4, the second embodiment is preferably the sameor similar to the first embodiment shown and described with respect toFIG. 2, except that the second embodiment shown at FIG. 4 utilizes aholographic beam sampler 48 (for example: HBS-series from GentecElectro-optics, Sainte-Foy, Quebec, Canada). Holographic beam sampler 48is preferably a transmissive diffraction grating formed on a transparentsubstrate (see above for the choice of materials transparent in the VUVrange). Advantages of the holographic beam sampler 48 include: (1) onlyvery small portion of the beam energy is split off (typically ˜0.1%),therefore, insertion losses are very low, e.g., as compared to ˜8% forconventional beam-splitter, and (2) wavelength separation is achieved atthe same time, since the diffraction angle for the red portion of thebeam is different from that of the VUV component, thus making the designsimple and robust. A disadvantage of the holographic sampler 48,however, is its higher cost. The choice of preferred materials for thediffractive beam sampler is dictated by its transparency in VUV rangeand radiation hardness. Examples of such materials are CaF₂, MgF₂,quartz, fused silica, doped fused silica, LiF, BaF₂.

[0056] The VUV portion of the beam 20 that is diffracted at theholographic beam sampler 48 is directed to a reflector 50 such as a VUVmirror or beamsplitter. The reflector directs the VUV light toward theassembly 40 and detector 38. The reflector 50 is designed for maximumreflectance at VUV wavelengths. The reflector 50 may be at least partlytransmissive at visible wavelengths to prevent or minimize red lightreflection towards the detector. A copper shield may be provided aroundthe reflector 50 to absorb this red light, e.g., so that the red lightis not otherwise reflected within the enclosure towards the detector 38.An example of such an arrangement of the reflector 50 is described atU.S. patent application Ser. No. 60/166,952, which is assigned to thesame assignee and is hereby incorporated by reference.

[0057] Referring to FIG. 5, the third embodiment is the same or similarto that shown and described with respect to the first embodiment of FIG.2, except that the third embodiment of FIG. 5 utilizes dichroicdielectric mirrors 52 in order to achieve separation of the VUV beamfrom the red portion of the laser output. In the third embodiment shownin FIG. 5, one beam-splitter 36 a and two dichroic mirrors 52 arepreferably used. The dichroic mirrors 52 are preferably formed bydepositing thin quarter-wave layers of dielectrics with alternating highand low refractive index, so that VUV beam is mostly reflected and redlight is almost completely transmitted. Other details of dichroicmirrors 52 are understood by those skilled in the art. Typically, acontrast ratio between the reflectance of the VUV light and the redlight of better than 30 can be achieved. The choice of the number ofmirrors is determined by the suppression ratio desired for reducing thesignal caused by the red component, e.g., below 1.0% or less. Twomirrors will typically provide at least two orders of magnitude contrastratio.

[0058] Referring to FIG. 6, the fourth embodiment is an alternativevariation of the first embodiment, and as such, is the same as orsimilar to the first embodiment of FIG. 2, except that the fourthembodiment of FIG. 6 includes only one beam splitter 36 a and a grating46. In the arrangement of FIG. 6, the intensity of the optical signal tothe detector 38 is increased compared to the first embodiment of FIG. 2.This may be desired if the sensitivity of the detector 38 is otherwiseinsufficient at a given output power of the laser. Alternatively, threeor more beamsplitters 36 a, 36 b, 36 c, etc., can be employed, with theadvantage is some circumstances that a reduction in signal to thedetector 38 may be achieved. In doing so, an advantage of reducing theintensity of the beam at the diffraction grating 46, and the assembly40, particularly including the collecting lens and attenuator grids, isthat the lifetimes of these components may be increased. At the sametime, decreasing in the intensity of the beam sample can lead to a lowersignal-to-noise ratio if the noise is dominated by scattered lightinside the housing of the energy detector 38. Therefore, depending onthe output power of the VUV laser and sensitivity of the detector 38,there is some optimum number of the beamsplitters 36 a, etc. that may beselected. These considerations apply to the second and third embodimentas well as to the first embodiment.

[0059]FIGS. 7a and 7 b show alternative arrangements of beam-splittersthat can be utilized in the first and third embodiments, respectively,preferably when the laser output is polarized. In both FIG. 7a and FIG.7b, two reflectors 36 a and 36 b, preferably both beamsplitters, areused. The reflectors 36 a and 36 b are aligned to eliminate deviationsdue to polarization fluctuations and preferences based on differingreflectivities for the orthogonal polarization components.

[0060] Generally, the first beam splitter 36 a and the second reflector36 b are aligned so that the polarization dependence of the reflectivityof the first beam splitter 36 a is compensated by the polarizationdependence of the reflectivity of the second reflector 36 b. Forexample, the first beam splitter 36 a may be aligned to reflect thep-polarization component of the incident beam at 10% of the efficiencyof the s-polarization component. The second reflector 36 b is thenaligned to reflect the component corresponding to the s-polarizationcomponent incident at the first beam splitter 36 a at 10% of theefficiency of its orthogonal counterpart corresponding to thep-polarization component incident at the first beam splitter 36 a. Thus,the overall dependence on the polarization of the output beam of thereflectivity of the first beam splitter 36 a-second reflector 36 bcombination is reduced or eliminated.

[0061] Preferably, the first reflector 36 a of both FIGS. 7a and 7 b isoriented in such a way that in the case that the incident laser beam ispolarized, the beam is p-polarized with respect to this firstbeam-splitter 36 a. The second reflector 36 b of both FIGS. 7a and 7 bis preferably oriented in a perpendicular plane to the first reflector36 a, so that the p-component of laser beam reflected from the firstreflector 36 a is s-polarized with respect to the second reflector 36 b.Preferably, each reflector 36 a and 36 b reflects the beam at anincidence angle of substantially 45 degrees.

[0062] An additional advantage of this configuration of the reflectors36 a and 36 b of both FIGS. 7a and 7 b is that the reflectivity of thefirst reflector 36 a for a polarized laser beam is significantlyreduced, typically from about 4% to 0.1%. Therefore, more of the beampower is available for the application. At the same time, the aboveexplained advantage of polarization selectivity of the first reflector36 a is compensated by the inverse selectivity of the second reflector36 b, since p- and s-components of the incident beam become s- andp-components, respectively, at the second reflector 36 b.

[0063] While exemplary drawings and specific embodiments of the presentinvention have been described and illustrated, it is to be understoodthat that the scope of the present invention is not to be limited to theparticular embodiments discussed. Thus, the embodiments shall beregarded as illustrative rather than restrictive, and it should beunderstood that variations may be made in those embodiments by workersskilled in the arts without departing from the scope of the presentinvention as set forth in the claims that follow, and equivalentsthereof.

[0064] In addition, in the method claims that follow, the steps havebeen ordered in selected typographical sequences. However, the sequenceshave been selected and so ordered for typographical convenience and arenot intended to imply any particular order for performing the steps,except for those claims wherein a particular ordering of steps isexpressly set forth or understood by one of ordinary skill in the art asbeing necessary.

What is claimed is:
 1. An excimer or molecular fluorine laser system,comprising: a discharge chamber filled with a laser gas mixture at leastincluding molecular fluorine and a buffer gas; a plurality of electrodesin the discharge chamber connected to a discharge circuit for energizingthe laser gas mixture; a resonator having the discharge chamber thereinfor generating an output beam; a sealed enclosure for providing a beampath for the beam that is substantially free of contaminant species sothat the energy of the beam can reach an application process withoutsubstantial disturbance due to the presence of the contaminant speciesalong said beam path; and at least one optical component within theenclosure, and wherein said beam interacts with said at least oneoptical component within said enclosure and is directed along a beampath within said enclosure that is protected from being substantiallydisturbed by said contaminant species, such that in operation of saidlaser system, said at least one optical component interacts with saidbeam which is directed from the at least one optical component alongsaid beam path within said enclosure and not substantially disturbed bysaid contaminant species.
 2. The laser system of claim 1, wherein saidat least one optical component includes a diffraction grating fordispersing said beam such that only a selected portion of a spectraldistribution of said beam continues to propagate along said beam pathand other portions of said spectral distribution of said beam aredispersed away from said beam path.
 3. A beam delivery system fordelivering a beam of an excimer or molecular fluorine laser, comprising:an enclosure for sealing a beam path from the outer atmosphere; at leastone port for preparing an atmosphere within said enclosure to maintainsaid enclosure substantially free of contaminant species, wherein saidenclosure contains at least one optical component therein forinteracting with the beam, and wherein said beam interacts with said atleast one optical component within said enclosure and is directed alonga beam path within said enclosure that is protected from beingsubstantially disturbed by said contaminant species, such that inoperation of a laser system having said beam delivery system coupledthereto, said at least one optical component interacts with said beamwhich is directed from the at least one optical component along saidbeam path within said enclosure and not substantially disturbed by saidcontaminant species.
 4. The beam delivery system of claim 3, whereinsaid at least one optical component includes a diffraction grating fordispersing said beam such that only a selected portion of a spectraldistribution of said beam continues to propagate along said beam pathand other portions of said spectral distribution of said beam aredispersed away from said beam path.
 5. The system of claim 3, whereinthe at least one port for evacuating the enclosure.
 6. The system ofclaim 3, wherein the at least one port for flowing an inert gas withinthe enclosure.
 7. The system of claim 6, wherein the at least one portfurther for evacuating the enclosure prior to flowing said inert gaswithin said enclosure.
 8. A sub-200 nm lithographic exposure system,comprising: a sub-200 nm lithographic exposure radiation source forgenerating sub-200 nm lithographic exposure radiation; a sealedenclosure for providing a beam path for the exposure radiation that issubstantially free of contaminant species so that the energy of theexposure radiation can reach an application process without substantialdisturbance due to the presence of the contaminant species along saidbeam path; and at least one optical component within the enclosure, andwherein said exposure radiation interacts with said at least one opticalcomponent within said enclosure and is directed along a beam path withinsaid enclosure that is protected from being substantially disturbed bysaid contaminant species, such that in operation of said exposuresystem, said at least one optical component interacts with said exposureradiation which is directed from the at least one optical componentalong said beam path within said enclosure and not substantiallydisturbed by said contaminant species.
 9. The laser system of claim 8,wherein said at least one optical component includes a diffractiongrating for dispersing said beam such that only a selected portion of aspectral distribution of said beam continues to propagate along saidbeam path and other portions of said spectral distribution of said beamare dispersed away from said beam path.
 10. A beam delivery system fordelivering exposure radiation generated by a sub-200 nm lithographicexposure radiation source, comprising: an enclosure for sealing a beampath for exposure radiation generated by said sub-200 nm lithographicexposure source from the outer atmosphere; at least one port forpreparing an atmosphere within said enclosure to maintain said enclosuresubstantially free of sub-200 nm photoabsorbing species, and whereinsaid exposure radiation is directed along a beam path within saidenclosure that is protected from being substantially attenuated by thepresence of said sub-200 nm photoabsorbing species, such that inoperation of a sub-200 nm lithographic exposure system including thebeam delivery system, said exposure radiation is directed along saidbeam path within said enclosure and not substantially attenuated due tothe presence of said sub-200 nm photoabsorbing species.
 11. The systemof claim 10, wherein the at least one port for evacuating the enclosure.12. The system of claim 10, wherein the at least one port for flowing aninert gas within the enclosure.
 13. The system of claim 12, wherein theat least one port further for evacuating the enclosure prior to flowingsaid inert gas within said enclosure.